Topic: '76 CB750K - Very low power at low RPM, flat accelleration

[Beatmalls]

Hey SOHC,

I have an issue with my '76 CB750K and I'm hoping someone can help me diagnose.

The issue is that when I open the throttle with the bike at a stop, the RPM's go down, way down, and will stall unless I rev it up to about 3k, feather the clutch for a bit as the bike gets rolling, and gets some momentum.  Then when it's going, it GOES but I have to keep the RPM up and it just doesn't have any real guts at all.  It can get up to speed, and it sounds like it's firing on all 4, but the power is not so good.

I had it off the road for 2 years, and when I was bringing it back around I did the standard maintenance per the shop manual, valves, plugs, timing/dwell, carb clean and balance, and things seemed good.  Compression is good and even across all 4 cylinders.  Battery is new and charged.

Carbs I think were not doing so well as I was riding before though, here's a shot of what they looked like after I rode it for a while after this first pass of getting it 'roadworthy'



Clearly it wasn't right.  #1 is carbon fouled - I have re-balanaced and dialed the air screws (there was smoke coming from #1 and after the adjustment, no smoke), so I'm hoping this is not still the case - but since performance hasn't changed, and since I've obviously been wrong before, maybe they're still way off.  The #4 image on the far right - is that "too lean / too hot"?

Just from the performance, I'd have suspected a dirty jet or something but I am sure that the carbs are clean.  I did have an issue with carbs leaking but I adjusted the floats (they aren't overflowing, but there is definitely gas in the bowls).  It was after this float adjustment that I cleaned the plugs and balanced the carbs again, and again got a good idle.  I'd also spent some time dialing the air screws to see if a bit of adjustment here or there would improve performance, but there's no appreciable change. 

When balancing the carbs both times, the vacuum was a bit low in 1 and 2, but I was able to adjust things into an OK state per the manual, and it was idling better than ever, so I was hopeful.  But when I actually put it in gear and try to GO, it just has no guts.  It does get up and go, but the power just isn't there.

At this point I'm out of ideas and need a bit of guidance, alternative ideas on what I might want to take a look at.  I've gone through the FAQ's and don't see anything that seems to fit this issue in particular.  Apologies for the somewhat convoluted post, I've tried to keep it thorough but perhaps this is resulting in too much info!

Thanks in advance SOHC!  You've been very helpful in the past.  Let me know if I can provide more info to help figure this thing out.

~Shane

[flybox1]

Please tell us your intake and exhaust setup, mixture screw setting, and what jets you are currently running.

[Beatmalls]

It has the standard air box, and stock 4 into 4 exhaust. 

I'm not 100% sure what the jets are but I can only assume they are standard because the majority of the bike is standard.

As far as mixture setting - I started off with 1.5 turns out on all 4 and adjusted a few out maaybe as much as 1 full turn (probably less) to deal with smoke from the exhaust, but no more than that.

[flybox1]

what color smoke? blue, black, water vapor?
if all is stock, set the idle mixture screws to stock, and clean your carbs.  make note of the jet sizes and brand.
unless you are the original owner, assume the PO changed them until you verify.
inspect and clean all jets and passageways of debris and clogs.
verify your carb floats are all the proper height.

[flybox1]

Assuming youve done the proper maintenance, I suspect its a air/fuel mixture issue (supported by your varying plug color)
Thorough carb cleaning
Check for correct (stock) jetting, mixture screws set at stock
Float height check
Clear tube test
Run bike. Check vacuum sync again.

[mrbreeze]

Even if your float height checks out....do the clear tube method. That way you're sure of fuel level. Make sure timing, points, valve adjustment are on spec. Lots of people say they've cleaned the carbs but come back and say they weren't thorough enough. They have to be "CLEAN" for them to work properly. What are you using to synch the carbs and are you doing it properly? If everything checks out good the bike should run well. Try fresh properly gapped plugs. Sometimes things get overlooked and will make you wanna pull your hair out but if you stick with it things will be right again.

[Beatmalls]

Slow on the reply - meetings!  Thanks for the replies everyone.

First off the smoke from the tailpipe didn't seem blue, more of a neutral color.  There wasn't THAT much smoke so it wasn't easy to tell, but it went away after I leaned the mixture via air screw.

I am not super experienced with this stuff so it's very likely I've just not done something well or thorough enough and that's what's causing the issue.  That being said, no I haven't tried the clear tube method for checking float height - have just popped off the bowls and looked inside.  That does seem a lot better - will give that a try.  I haven't done it before though, if anyone could point me towards any specific pages on how to do it, that'd be sweet, but I can just use the power of google to figure out what I need to do.  Will scour the FAQ but I'm not sure I've seen anything regarding that there.

I did the valves and can only say they are as good and on spec as I've ever had them.  I'm doing the carb balance with a "carbtune pro" (purchased when I was a richer man) and I'm pretty sure I'm doing it right!  I mean I've done it before to success so I can only say that it's as close to the right way that I know how to do.

I think I'll shell out for a new set of plugs so I know things are at a good starting point

Besides that, from the sounds of it, everyone is thinking carbs.  So even though MAN I'm sure they're clean as a whistle, I guess I'll just crack them open and triple check that it's all good and there's no blockage or gummy stuff in there, see that it's all working A-OK, then do the float height via clear tube method.  Who knows, maybe some debris or something go is caught in one of the jets, My tank is spotless (on the inside) but anything can happen. 

Thanks again for the replies folks, it really helps.

[flybox1]

Set ALL of your mixture screws back to stock setting when you have your carbs off the bike.
Clear tube test is easy.
Taper and insert clear tubing into drain screw hole, secure it in place, and then turn on fuel.  see where it lands. 
Adjust float height only if necessary.
do all of this with the carbs OFF the bike, rig up an aux fuel supply, and when fuel levels are all correct, reinstall carbs on bike.


for PD carbs, the tube is installed on the overflow pipe

[Beatmalls]

Great thanks for the breakdown, and pointing out that you can do it off the bike!  Much more convenient.
Hopefully I'll be back next week with a followup success post!  We'll see.  Time is limited and holidays exacerbate the issue!

[mrbreeze]

I did mine on the bike. It was on the centerstand and level. I don't see any reason to not do them on the bike?

[seanbarney41]

Did you confirm that the ignition advance is correct?

[Mr. Mike]

"Had it off the road for two years".
You don't mention it, but fresh oil and filter yes?

[Don R]

Detail the carb cleaning, were the emulsion tubes all clear and all the passages able to pass carb cleaner? Does the petcock flow enough fuel? Check the petcock bowl for rust or debris. Inline filters can plug even when they appear spotless.

 Does it get better or worse with the choke on? Oh and good luck.

[flybox1]

I did mine on the bike. It was on the centerstand and level. I don't see any reason to not do them on the bike?

who wants to wrestle carbs on and off multiple times when you need to clean the carbs to begin with, and possibly make many small adjustments in float heights, hook up fuel, tank on tank off, clear tube test multiple times, etc. etc.
take carbs off once, rig up a stand, do all testing and adjustments off the bike, put carbs back on once.  itll save time and frustration.

[Beatmalls]

Quick follow up.  I'm not out of the woods yet but I finally got (and modified) the connectors and tubes and checked out/adjusted the effective float height.  The level was really low in a few, and somewhat low in a few others, so I'd expect some improvement when I do fire it up.  Here's where I got them all to line up, roughly.  Haven't started it up yet - the task ended late and didn't want to wake the neighbors.  When I do so I'll post what I find.



In a likely vain attempt to to save work, I did them ON the bike - in case the float height is the main culprit behind my issues.  I say it was likely in vain because there were some dark dirty bits in the gas that came out of the carbs when drained, so in all likelihood there is an issue with the jets so I'll have to go in and clean them anyway, making this adjustment moot.  At any rate it's back together now and I'm going to just see what things are like.  If they aren't immediately improved, I'll go back and do the full monty.

Also for the record I got a new set of plugs and have set the air screws back to default (1.5 turns out).  Will replace oil and filter when the filter arrives in the mail!

Thanks again for all the replies, this thread has gotten me back on track with the bike and I've been having a lot of fun getting dirty.

~s

[flybox1]

Glad youre making progress.  Best idea is to tape those tubes in place.
You might notice if you move them up or down the reading slowly changes.
IIWM, I'd say the fuel level you've pictured is low. 
Raise the level by 1/2 the distance between its current level and the bottom of the carb body.

the dirty bits in your fuel have to come from somewhere.  shake and drain your tank into a mason jar.
see what comes out.
a new tank filter, or better yet, a good cleaning of your tank, might do the trick.

[mrbreeze]

I agree on the level. Should come up a bit. 3mm from the top edge of bowl. Fuel level is more critical than one might believe. Also agree on clearing debris out of tank. Any crap in the tank can't help but to make it's way to your freshly cleaned carbs and plug them up. If there is rust in the tank you need to get it all out......not just what will come out with a flush.

[Beatmalls]

Aha I see, I knew it'd be valuable posting the actual level!  It seemed good to me but if it's gotta come up, it's gotta come up!

I don't have any pics but the tank looks clean and rust free, as best as I can tell (clearly I've been wrong in the past).  I'll have another closer look to be sure, but another reason for thinking it's in the carbs causing the "gunkening" is that not all the bowls had gunk in them when I drained them out - some were noticeably shiny and the fuel was clear (others, not so much).... so I would expect the issue is within the carbs themselves rather than the tank.   

I can only imagine then, that there's a good chance the jets are not as clean as I'd thought. I'll probably just take them off, do a proper cleaning, and get the float set to what you guys are saying, before I put them back on.  The gunk is nagging at me, and I don't want to go and do the float and tune/balance, knowing I'll probably have to do it again. 

Question though - I don't have a fuel filter on there - it's never had one.. I am guessing it should, even if the tank is clean, stuff might get in there.. these ones seem common on some parts sites I frequent, seems like it'll do the job, but LMK if you think otherwise http://www.amazon.com/Emgo-14-34451-Glass-Fuel-Filter/dp/B0022ZUJGG/ref=sr_1_2?ie=UTF8&qid=1449007491&sr=8-2&keywords=emgo+universal+fuel+filter

In response to an earlier question about jet sizes, a while back I used these kits to rebuild two of the carbs:
http://www.oldbikebarn.com/honda-cb750-k-four-71-76-carb-kit
This was long before the bike was in the hobbled state it's in now.  I only rebuilt 1 and 2 with these, 3 and 4 were in much better shape, so I kept them as is.  The bike ran really nicely for a while like this, but now I'm not so sure if it's contributing to my issues.  I guess my expectation was that these were made to match the original.  The jets sure LOOK like the other ones - though the air screws look different. Very little information on the listing (probably not the smartest thing to buy, but since it was a kit I probably just figured it would be "right").  In a perfect world I guess I'd go and get two more kits but I'd really like to avoid dropping fifty bucks on a maybe.  I'll take a closer look at the jets when I've got it all apart and see if I can glean some more information from them.

[HondaMan]

Just to reduce one of your variables: the air screws must all be set at 1 turn out. Their effective operating range is 7/8 to 1-1/8 turn, beyond which they make no further difference.

Most of the time, I find this sort of problem is either sticky float valves that let the bowls run dry (or low) or else the pilot jets are plugged or partially plugged. The passages are tiny, so use tiny wires to clean them out. I get many carbs for cleaning that sat 2-3 years, and most of those have one of more plugged pilot jets, and usually stuck float valves, from the goo in the brass seats and calcides on the moving valve itself (scrape lightly with a razor blade to clean it off).

[mrbreeze]

Without even looking at your links I see some heavy negatives.......Old Bike Barn & Emgo. There are many horror stories here with those 2 names........do a search. I would recommend getting a proper stock strainer type fuel filter as opposed to any type of inline or running sans filter. It really doesn't take much to plug the small holes of your emulsion tubes and jets. In line filters can impede proper fuel flow also as can bad fuel line routing.

[Beatmalls]

Aha I see, glad I asked!  I'll avoid those emgo thingies then. It appeared innocuous to my untrained eye.

Actually that brings to mind a story. After one particular incident with OBB I stopped buying stuff from them, but it was more to do with the website than with what they sell.  I was doing a pass at tightening up my cyber security and I wanted to change my password with their site to something more secure.  There was no link to change the PW so I emailed their support, and instead of the usual automated redirect, they just emailed me back asking "Ok what do you want your new password to be?"  Which is...odd.. and just says to me that their security is likely pretty flimsy, if they want my plain text password sent to them via email.  Plenty of places to get this stuff so why bother with a place that might leak my info.

At any rate, if anyone has a recco on where I should be looking to buy these things (carb rebuild kits, fuel filter), I'm all ears.  Function is most important but yes for me right now, cost is a factor, so good deals are good.   I tend to look for things on bike bandit since it has the exploded schematic and I can see that I'm buying the right thing.. but what LOOKS like the fuel filter here:

http://www.bikebandit.com/oem-parts/1976-honda-cb750k/o/m9456#sch400770

is the 'strainer set' (#3), correct?  Thirty bucks hrm.  Cheaper would be nice.

[mrbreeze]

I couldn't get your link to load so I did it myself. The strainer I saw there was #3 and their price was $26. Pricey but probably cheaper than the dealership. It's a part you can clean (carefully) and reuse so it's a good investment. I got my carb kits from Dennis Kirk but that was 10 years ago. There are lots of good companies to get parts from.....Z1 enterprises, Yamiya, David Silver, Service Honda. There is also a good feature on this site that lists good sellers,bad sellers. This comes in handy when looking for stuff that you may want to buy. If you aren't sure...ask about what you want before you pull the trigger and someone here will either give you a thumbs up to buy or steer you clear.

[HondaMan]

Aha I see, glad I asked!  I'll avoid those emgo thingies then. It appeared innocuous to my untrained eye.

Actually that brings to mind a story. After one particular incident with OBB I stopped buying stuff from them, but it was more to do with the website than with what they sell.  I was doing a pass at tightening up my cyber security and I wanted to change my password with their site to something more secure.  There was no link to change the PW so I emailed their support, and instead of the usual automated redirect, they just emailed me back asking "Ok what do you want your new password to be?"  Which is...odd.. and just says to me that their security is likely pretty flimsy, if they want my plain text password sent to them via email.  Plenty of places to get this stuff so why bother with a place that might leak my info.

At any rate, if anyone has a recco on where I should be looking to buy these things (carb rebuild kits, fuel filter), I'm all ears.  Function is most important but yes for me right now, cost is a factor, so good deals are good.   I tend to look for things on bike bandit since it has the exploded schematic and I can see that I'm buying the right thing.. but what LOOKS like the fuel filter here:

http://www.bikebandit.com/oem-parts/1976-honda-cb750k/o/m9456#sch400770

is the 'strainer set' (#3), correct?  Thirty bucks hrm.  Cheaper would be nice.

For carb kits and parts, the latest word has been 'trouble' with some aftermarket jets out there. Someone is cloning them improperly and selling them as 'genuine'. The good news is: jets don't wear out, nor have your slide needles. About the only parts that need some replacement once in a while in the K6 carbs are the bowl gaskets, the O-rings between the carbs (where the fuel flows into them, if they leak), and after MANY miles, the float valves and seats (together). Even the latter have some issues now, in particular they now come with stiffer springs in them than what they used to be new. So, this makes the float bowl levels sit lower at highway speed, causing loss of some midrange power. You can comp for this by making the bowls deeper (use 25-24mm instead of 26mm setting), so they still work oK overall. The ones for your K6 have tiny strainer-filters on top of them, up inside the fuel port.

...which brings up a possibility: have you unscrewed one of them to see if their little filter screen is plugged with grit?

[Beatmalls]

Quick reply:
Have not checked for grit in the screens, will do!  And will be sure to check the approved sellers list and checking in with the forums before dropping the hammer on any purchases!
Amazing support here from you guys, really hard to imagine a more helpful community.  I just need to find more time to follow up on all this advice!  My free time comes in small and unpredictable bursts.
First thing's first, get those things off and open and see what's going on inside.

[Beatmalls]

And one more followup to this - I might take a look at the old jets and just use the float valves and seats.  As I am now recalling, the float valves and seats are what was in bad enough shape for me to go in and buy new ones - pretty sure I only did the whole kit together because I felt like "rebuild" seemed more of a sure bet for longevity than just replacing the one faulty piece.  As I'm gathering, I was probably wrong, so if the old jets look salvageable, I'll put those back on and just retain the gaskets and float valves/seats.